As the world moves towards higher performance communication and computer systems, microelectronic devices are running into basic barriers related to heat dissipation. Options under exploration include active and passive packaging designs. The active designs require fans or pumps to circulate fluids for heat extraction, often leading to heat generation, power drain, and new failure modes. The alternative is to move into passive designs, such as fins to radiate heat into the enclosure.
High thermal conductivity materials are desirable for heat dissipation, and current favorites include tungsten-copper, molybdenum-copper, and aluminum or copper. The latter choices suffer from high thermal expansion coefficients which induce a new failure mechanism through thermal fatigue, associated with turning on (heating up) and turning off (cooling down) an electronic device. In order to sustain the desired thermal expansion match with silicon while maximizing thermal conductivity, the top materials then tend to be heavy, expensive, and modest in thermal conductivity. Only diamond provides a high thermal conductivity with low thermal expansion, but its cost is prohibitive.
In recent years, we have made progress in designing improved functionality into a structure through the combination of two different materials using a process termed two-material powder injection molding. This step toward functionality directly built into a device has potential benefits in microelectronic packaging. The walls might be fabricated from a good glass-sealing alloy, such as kovar, while the base would be fabricated from a low thermal expansion material, such as tungsten-copper. However, even these two-material combinations are limited by the thermal conductivity of the base. Currently, tungsten-copper is capable of thermal conductivities in the 200 W/m/K range. This is still half that possible with pure copper, but, again, still fails to satisfy the thermal expansion requirement. We note that diamond can achieve 2000 W/m/K.
As will become clear later, the present invention makes use of two-material powder injection molding to implement a different approach to this problem. This process has been described in application Ser. No. 09/733,527 Dec. 11, 2000 “Method to form multi-material components”. Briefly, this process shows how powder injection molding may be used to form a continuous body having multiple parts, each of which has different physical properties such as magnetic characteristics or hardness. This is accomplished through careful control of the relative shrinkage rates of these various parts. Additionally, care is taken to ensure that only certain selected physical properties are allowed to differ between the parts while others may be altered through relatively small changes in the composition of the feedstocks used.
A routine search of the prior art was performed and the following U.S. Patents were found to be of interest:
U.S. Pat. No. 6,410,982 (Brownell et al.); U.S. Pat. No. 6,321,452 (Lin); U.S. Pat. No. 6,385,044 (Colbert at al); U.S. Pat. No. 6,370,749 (Tseng et al.); U.S. Pat. No. 6,303,191 (Henne et al.); U.S. Pat. No. 6,293,333 (Ponnappan et al.); U.S. Pat. No. 6,230,407 (Akutsu); and U.S. Pat. No. 6,070,654 (Ito).
Additionally, the following publications were discovered during our search:
1. B. R. Babin, G. P. Peterson, and D. Wu, “Steady-State Modeling and Testing of a Micro Heat Pipe,” Journal of Heat Transfer, vol. 112, August 1990, pp. 595-601.
2. J. P. Longtin, B. Badran, and F. M. Gerner, “A One-Dimensional Model of a Micro Heat Pipe During Steady State Operation,” Journal of Heat Transfer, vol. 116, August 1994, pp. 709-715.
3. L. W. Swanson, “Heat Pipes,” The CRC Handbook of Thermal Engineering, F. Kreith (ed.) CRC Press, N.Y., 2000, pp. 4.419-4.429.